The host galaxies of radio-loud active galactic nuclei: colour structure

We construct a sample of 3516 radio-loud host galaxies of active galactic nuclei (AGN) from the optical Sloan Digital Sky Survey and Faint Images of the Radio Sky at Twenty cm. These have 1.4-GHz luminosities in the range 1023-1025 W Hz−1, span redshifts 0.02 < z < 0.18, are brighter than r*petro < 17.77 mag and are constrained to 'early-type' morphology in colour space (u*−r* > 2.22 mag). Optical emission line ratios (at >3σ) are used to remove type 1 AGN and star-forming galaxies from the radio sample using Baldwin-Phillips-Terlevich diagnostics. For comparison, we select a sample of 35 160 radio-quiet galaxies with the same r*-band magnitude-redshift distribution as the radio sample. We also create comparison radio and control samples derived by adding the NRAO VLA Sky Survey (NVSS) to quantify the effect of completeness on our results. We investigate the effective radii of the surface brightness profiles in the SDSS r and u bands in order to quantify any excess of blue colour in the inner region of radio galaxies. We define a ratio R=re (r)/re (u) and use maximum likelihood analysis to compare the average value of R and its intrinsic dispersion between both samples. R is larger for the radio-loud AGN sample as compared to its control counterpart, and we conclude that the two samples are not drawn from the same population at >99 per cent significance. Given that star formation proceeds over a longer time than radio activity, the difference suggests that a subset of galaxies has the predisposition to become radio loud. We discuss host galaxy features that cause the presence of a radio-loud AGN to increase the scale size of a galaxy in red relative to blue light, including excess central blue emission, point-like blue emission from the AGN itself and/or diffuse red emission. We favour an explanation that arises from the stellar rather than the AGN light.